Surface Smoothing by Atomic Layer Deposition and Etching for the Fabrication of Nanodevices

In many nano(opto)electronic devices, the roughness at surfaces and interfaces is of increasing importance, with roughness often contributing toward losses and defects, which can lead to device failure. Consequently, approaches that either limit roughness or smoothen surfaces are required to minimize surface roughness during fabrication. The atomic-scale processing techniques atomic layer deposition (ALD) and atomic layer etching (ALE) have experimentally been shown to smoothen surfaces, with the added benefit of offering uniform and conformal processing and precise thickness control. However, the mechanisms which drive smoothing during ALD and ALE have not been investigated in detail. In this work, smoothing of surfaces by ALD and ALE is studied using finite difference simulations that describe deposition/etching as a front propagating uniformly and perpendicular to the surface at every point. This uniform front propagation model was validated by performing ALD of amorphous Al2O3 using the TMA/O2 plasma. ALE from the TMA/SF6 plasma was also studied and resulted in faster smoothing than predicted by purely considering uniform front propagation. Correspondingly, it was found that for such an ALE process, a second mechanism contributes to the smoothing, hypothesized to be related to curvature-dependent surface fluorination. Individually, the atomic-scale processing techniques enable smoothing; however, ALD and ALE will need to be combined to achieve thin and smooth films, as is demonstrated and discussed in this work for multiple applications.


S.A Inherent roughness on smooth Si substrates
ALD of Al 2 O 3 on a rough surface leads to smoothing, however when deposited on a very smooth surface, ALD of Al 2 O 3 is observed to lead to an increase in roughness, as is shown in Figure S.1.
In literature an increase in roughness due to ALD of Al 2 O 3 has also been observed. 1,2 The observed roughness is not caused by crystallinity, as the deposited film is amorphous, instead, it has been hypothesized that the roughening effect is caused by random deposition. 3 The roughness that forms after ALD on a smooth surface, is inherent to the specific deposition process or process conditions (i.e., deposition temperature, purge times, dose times etc.) and is therefore referred to as the inherent roughness.     highlights the region that has been converted into the pixel map shown on the right. The rise/fall parameter is also highlighted on the pixel intensity map.

S.C PSD noise correction
Surfaces as measured by AFM always have a certain amount of electronic noise. This noise can be corrected for by two methods. The first method is to measure the noise of the AFM measurement S9 itself, which is done by setting the measurement size to something very small (e.g., 1 nm). The AFM that is used cannot measure features this small, so only measurement noise is observed. The noise is assumed to be independent of the measurement area, meaning that for different measurement sizes, the PSD is identical. Subtracting the noise PSD from the measured PSD results While for high roughness surfaces the second method is most appropriate, where the noise from the AFM measurement is negligible, as can be seen from the good agreement at low wavenumber in Figure S.6b, and noise only has an impact at high wavenumbers. The first method was thus used for the ALE cases, as the roughness was generally lower. The second method was used for the ALD cases, in which instance a value of 1·10 -28 m -3 gives the best results. For an ideally conformal ALD process, or an ideally isotropic etch process, the propagation rate F is equal to 1, which gives the equation from the work of Alasaarela et al.. 7 In this case the front propagation is monotonic/isotropic/uniform, i.e. equal everywhere on the surface.

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For the ALE process studied in this work, the propagation is not uniform, meaning that F is not constant, but is dependent on the mean curvature K(x,y) by F=1-εK. The mean curvature is the mean of the two principle curvatures. Using Euler's theorem (of differential geometry), it can be proven that the mean curvature is equal to the mean of any two orthogonal curvatures (x and y planes in this case).

S.E Smoothing rate for ZnO/Al 2 O 3 stack
Smoothing rate values were also determined for the dataset presented in section 4.3, as listed in smooth. Comparing both 35 nm ALD layers, we see that the rate for layer 6 is nearly double that S13 of layer 2 (ALD), which again could be explained by the higher starting roughness for layer 6.
This confirms that when depositing very thin layers, the roughness of the starting surface plays an important role in the rate of smoothing that is achieved. Table S1. Process used for each layer deposited/etched in Figure 7 with the film thickness from ellipsometry, RMS roughness and smoothing rate from AFM, and the roughness determined from TEM.

S.F AFM maps post ALE
Similar to ALD the uniform front propagation + curvature dependent (UFP+CD) can be used to predict how a surface will smooth during ALE. Using a measured starting surface as the input for the model it can be seen that there is good agreement between the modelled and experimental data.

S.G Optical properties of Al 2 O 3
Optical constant n is shown for a 35 nm Al 2 O 3 film grown at 300 ˚C fitted using a Cauchy model.
The extinction co-efficient k was zero over the entire wavelength range investigated here Figure S8: Refractive index for plasma ALD grown Al 2 O 3 . S16